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GCN Circular 13123

Subject
GRB 120327A: Swift detection of a burst with an optical counterpart
Date
2012-03-27T03:17:21Z (12 years ago)
From
Scott Barthelmy at NASA/GSFC <scott@milkyway.gsfc.nasa.gov>
B. Sbarufatti (INAF-OAB/PSU), S. D. Barthelmy (GSFC),
N. Gehrels (NASA/GSFC), E. A. Hoversten (PSU),
N. P. M. Kuin (UCL-MSSL), F. E. Marshall (NASA/GSFC),
D. M. Palmer (LANL), M. H. Siegel (PSU), M. Stamatikos (OSU/NASA/GSFC)
and T. N. Ukwatta (MSU) report on behalf of the Swift Team:

At 02:55:16 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 120327A (trigger=518731).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 246.828, -29.415, which is 
   RA(J2000)  =  16h 27m 19s
   Dec(J2000) = -29d 24' 54"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows 3 main peaks
with a total duration of about 60 sec.  The peak count rate
was ~6500 counts/sec (15-350 keV), at ~38 sec after the trigger. 

The XRT began observing the field at 02:56:32.2 UT, 75.6 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 246.86308, -29.41514 which
is equivalent to:
   RA(J2000)  = 16h 27m 27.14s
   Dec(J2000) = -29d 24' 54.5"
with an uncertainty of 3.6 arcseconds (radius, 90% containment). This
location is 110 arcseconds from the BAT onboard position, within the
BAT error circle. This position may be improved as more data are
received; the latest position is available at
http://www.swift.ac.uk/sper. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (1.40 x
10^21 cm^-2, Kalberla et al. 2005), with an excess column of 1.9
(+1.51/-1.37) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 5.41e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 250 seconds with the U filter starting
140 seconds after the BAT trigger. There is a candidate afterglow in the
rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	16:27:27.47 = 246.86445
  DEC(J2000) = -29:24:53.8  = -29.41494
with a 90%-confidence error radius of about 0.64 arc sec. This position is 3.6
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.92 with a 1-sigma error of about  0.15. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.34. 

Burst Advocate for this burst is B. Sbarufatti (boris.sbarufatti AT brera.inaf.it). 
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/too.html.)
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